1. Field of the Invention
The present invention relates to a rocking chair whose seat taken by a chair occupant can rock on a point above the seat.
2. Description of the Related Art
A rocking chair having a seat taken by a chair occupant performing arc motion around a point above the seat, preferably the point located near a gravity point of the occupant's body, has been developed for the purpose of, for example, recovery of tired lumbar muscle after long-term seating, prevention or treatment of lumbago, and lumbar muscle training in rehabilitation.
Such rocking chair is disclosed in JP 2-51540 U. The chair in this reference literature has a seat, a curved upper plate for supporting the seat which is provided on the lower side of the seat and has a convex surface facing downwardly, and a curved base plate for guiding the upper plate which is located below the upper plate and has a concave surface corresponding to the convex surface of the upper plate. The upper plate can slide on the base plate to rock on a point above the seat. A preferred embodiment described in the reference literature has ball casters provided on the concave surface of the base plate in order to smooth the rocking motion of the upper plate. The upper plate in the embodiment rocks on the ball casters.
In JP 7-503392 A, there is disclosed another active dynamic seat (rocking chair) having a seat, a dish-shaped seat shell which is provided on the lower side of the seat and designed to be convex in the downward direction, an intermediate member having a head part with a bearing engaging the seat shell so as to enable the seat shell to rock, and a support member connected to the intermediate member. The bearing has a plurality of balls rotatably supported in one cage or plural cages. That is, the bearing has a plurality of ball casters. The seat shell can move on the ball casters to rock on a point above the seat.
According to the rocking chair, as disclosed in the above reference literatures, comprising a projecting member provided on the lower side of the seat and having a convex surface forming a part of a spherical surface centered on a point located above the seat (the upper plate in the chair of JP 2-51540 U, the seat shell in the chair of JP 7-503392 A), and a guide member in contact with the convex surface of the projecting member and guiding the projecting member such that the projecting member can rock along the spherical surface (the base plate or the ball casters in the chair of JP 2-51540 U, the ball casters in the chair of JP 7-503392 A), the seat can freely rotate and can tilt in the direction of a force applied by a chair occupant.
Thus, the occupant of such chair can sway or twist his or her lumbar part on the seat. Further, efficient recovery of tired lumbar muscle, prevention or relaxation of lumbago, or lumbar muscle training in rehabilitation is achieved by using such chair, because the occupant's lumbar muscle is always exercised when he or she returns the seat in neutral position in order to maintain balance. Additionally, safe and trouble-free use is ensured even for a first-time user, since rocking of the seat is not accompanied with shift of a gravity point of the user sitting on the chair. As far as the present invention, the word “neutral position” means the position occupied by each component (the seat, for example) of a rocking chair without a circumferential external stress on the seat.
However, the rocking chair as disclosed in JP 2-51540 U having, as the guide member, the base plate with the concave surface corresponding to the convex surface of the projecting member does not have little success in smooth rocking motion of the seat due to friction induced between the concave surface and the convex surface. The use of the ball casters as the guide member also results in insufficient rocking motion smoothness, the reason is that deformation of the projecting member and the ball casters, which is caused by body weight of the occupant sitting on the seat, increases friction resistance in the rotating motion of the balls and the sliding motion of the projecting member on the balls.
Another problem is very big noise caused by the balls-rotation of the ball casters. This over loud noise prevents continuation of works such as deskwork and rehabilitation, when plural persons use the chairs having the ball casters in one room. In order to solve the problem of the big noise, lift of the seat by magnetic or hydraulic equipment might be effective solution. But, only a big and expensive rocking chair is manufactured by this solution. Highly-refined processing for obtaining exact spherical surfaces also makes the chair expensive